The present invention relates to a method for identifying a blockage of one or more spray openings of a dosing valve of an SCR catalytic converter system. The invention also relates to a computer program which performs all of the steps of the method according to the invention when executed on a computer. The invention also relates to a computer program product with program code, which is stored on a machine-readable carrier, for carrying out the method when the program is executed on a computer or control unit.
To comply with ever more stringent exhaust-gas legislation, it is necessary to reduce nitrogen oxides (NOx) in the exhaust gas of internal combustion engines, in particular of diesel engines. For this purpose, it is known to arrange an SCR (selective catalytic reduction) catalytic converter in the exhaust-gas region of internal combustion engines. In an SCR catalytic converter, nitrogen oxides in the exhaust gas of the internal combustion engine are reduced, in the presence of a reducing agent, to form nitrogen. In this way, the fraction of nitrogen oxide in the exhaust gas can be reduced considerably. For the reaction to proceed, ammonia (NH3) is required which is admixed to the exhaust gas. As reducing agents, use is thus made of NH3 or reagents which release NH3. In general, for this purpose, use is made of an aqueous urea solution (AUS) which is injected into the exhaust tract downstream of the SCR catalytic converter. Ammonia, which acts as a reducing agent, is formed from said solution. A 32.5% aqueous urea solution is commercially available under the trade name AdBlue®.
A dosing system for an SCR catalytic converter according to the prior art comprises a reducing agent tank, a delivery module and a dosing valve. The reducing agent solution is transported from the reducing agent tank to the dosing valve by the delivery module. The delivery module comprises a pump which is connected by means of lines to the dosing valve and if appropriate to a reducing agent return line. Said line system is referred to as hydraulic system.
The “California Air Resources Board” (CARB) demands the identification of a dosing quantity deviation in the catalytic converter system which leads to the exceedance of a defined nitrogen oxide emissions limit of the exhaust system of a diesel engine. This may occur in particular as a result of blockage of one or more spray openings of the dosing valve of an SCR catalytic converter system. It is thus necessary to make a distinction between demanded dosing mass, and the dosing mass actually dosed, of the reducing medium AdBlue®. In particular as a result of the provision of a reducing agent return line in the delivery module of the SCR catalytic converter and owing to the influence of the air volume enclosed in the system on pressure profiles during the dosing of certain masses of the reducing agent AdBlue®, an identification of dosing quantity deviations is not possible by evaluation of the gradient of pressure profiles in the hydraulic system or from a deviation in the pump rotational speed.
To identify dosing quantity deviations of the aqueous urea solution owing to a blockage of one or more spray openings of a dosing valve, the use of an additional mass flow sensor in the hydraulic system of the SCR catalytic converter is therefore necessary.